This invention relates to a method of fabricating a mesa stripe of a semiconductor wafer and, more particularly, to a method of producing an inverted mesa stripe having a waist portion narrower than the upper section of an ordinary mesa stripe; namely, a mesa stripe having an inverted trapezoidal cross section.
Mesa etching is commonly applied to a semiconductor wafer in, for example, manufacturing wave guides of optelectronic device. A semiconductor laser, in particular, needs to have a mesa stripe formed by mesa etching.
FIG. 1 is a schematical, oblique view of an ordinary embedded type semiconductor laser. The laser comprises a semiconductor (crystal) substrate 11, an active layer 12, a cladding layer 13, an ohmic contact layer 14, and embedded PN junctions 15, 16 and 17. The region occupied by active layer 12 is an embedded section. Striped mesa etching is applied to active layer 12 and cladding layer 13. To reduce resistance in the electrode contact, there is formed a so-called inverted mesa stripe, whose upper plane has a width W.sub.0 greater than the width .DELTA.W of the mesa waist section.
The width of embedded active layer 12 noticeably governs the property of the semiconductor laser. It is therefore necessary to control the embedded width of active layer 12 (substantially the waist width .DELTA.W of the mesa stripe) precisely. However, the width .DELTA.W of the inverted mesa stripe waist section can not be observed from above. Therefore, a process is employed whereby to observe the waist width .DELTA.W from the cross section of the vertically cut mesa stripe. Since, however, this process makes it necessary to cut away part of a semiconductor wafer, limitation is imposed on the frequency of observing said waist width .DELTA.W. Consequently, it is difficult to achieve the full observation of said waist width .DELTA.W and, subsequently, its control.
Recently, a process involving the application of a reference mesa (disclosed in IEEE Journal of Quantum Electronics, Vol, QE-20, No. 8, August, 1984, P855) has been proposed for controlling the waist width of the above-mentioned mesa stripe. In this process, a reference mesa stripe (monitor mesa stripe) narrower by a prescribed waist width than the prescribed mesa stripe (main mesa stripe) is arranged parallel with said main mesa stripe, and both the main mesa stripe and said reference mesa stripe are etched. The etching is stopped when the waist width of said reference mesa stripe is reduced to zero, namely, when said reference mesa stripe is cut off. Thus is produced a main mesa stripe having a prescribed waist width. This process has the merit of allowing one to determine whether the main mesa stripe has attained the desired waist width, simply by permitting observation of the main mesa stripe from above without the need to cut away part of the semiconductor wafer.
However, the above-mentioned proposed process is accompanied with the undermentioned drawbacks. The monitor mesa stripe extends from end to end of the semiconductor wafer plane. To cut off the monitor mesa stripe, therefore, etching has to be continued until the entire waist width of the monitor mesa stripe is reduced to zero. The different portions of the monitor mesa stripe are etched at different speeds. The center portion of the monitor mesa stripe may fail to have its waist width reduced to zero due to the irregular etching. If this happens, the monitor mesa stripe is not removed, and observation of the waist width of the corresponding main mesa stripe is obstructed. Further when the monitor mesa stripe happens to break, the waist width of the main mesa stripe can reach a prescribed width in the region where etching proceeds most slowly, but becomes narrower than the prescribed width in regions where etching is carried out faster. Further it is impossible to detect that region of the main mesa stripe whose waist width has reached a prescribed level. Therefore, the process disclosed in IEEE Journal of Quantum Electronics also presents difficulties in providing a mesa stripe having a desired waist width for the whole of its length. The above-mentioned problems are most noticeable when the etching solution is stirred.